1. Field of the Invention
The present invention relates generally to flexible wings used on various craft for lift or propulsion and more particularly pertains to devices for modifying and controlling the shape of such wings in order to increase power and enhance stability and control.
2. Description of the Prior Art
The efficiency and controllability of an aerodynamic surface is extremely sensitive to relatively minor changes in its shape. Moreover, shape requirements change as a function of airflow velocity and angle of attack. The problem of achieving an optimized shape is compounded in flexible wing designs wherein both surfaces are called upon to interchangeably fulfill windward as well as leeward functions.
Flexible wings are commonly utilized on sailing craft and a large variety of equipment and techniques has been developed to either directly or indirectly control the shape of the sail. The cut of the sail along with the type of materials utilized in its construction have a fundamental effect on the sail's ultimate shape while additional devices such as booms, sprits, spars, gaffs, outhauls, downhauls, boomvangs, etc. are utilized to modify the shape of the sail in order to accommodate various conditions. Relatively recent advances exploit the flexibility of specially constructed mast systems to further help shape the sail.
Two parameters that relate directly to a sail's shape and hence performance, are camber and twist. Camber refers to the convexity, arching or curving of the sail's surface relative the chord. The sail's camber, both in terms of shape and thickness, affects the range of angle of attack in which the sail can generate lift. The magnitude of the greatest deviation of the sail surface from the chord is a critical factor as is the point along the chord at which the greatest such deviation occurs. These two aspects of camber along with the profile of the "entry curve" of the sail's leading edge and the taper of the sail's trailing edge all affect the amount of power a sail is capable of extracting from air flowing thereover and the angles of attack at which such power can be extracted. Additionally, the sail's camber shape influences the boom center of pressure (bcp). The specific location of the bcp affects the control of the craft powered by the sail, while a shifting bcp causes instability. Due to the flexible nature of a conventional sail, varying magnitudes of airspeed and angle of attack typically alter the camber shape with a commensurate shift in the bcp.
The term "twist" is used to describe any variation in the angle with which the sail's chordlines emanate from along its leading edge. A chordline's orientation determines the angle of attack and while a substantial angle of attack may be necessary in order to extract power from an impinging air flow, too large an attack angle near a sail's head generates a disruptive and drag inducing vortex. It is therefore desirable to induce twist in the sail such that the angle of attack is greatest in the sail's "working section" (the bottom third to one half) while tapering off to near zero at its head. Twist additionally affects the spanwise center of pressure (scp). In a fashion similar to the bcp, the scp affects the control and stability of the craft powered by the sail.
Applicant has previously developed various "camber inducing devices" (see U.S. Pat. Nos. 4,686,921; 4,708,079; and 4,856,447) that serve to modify the shape of the sail by directly coupling a sail's battens with the mast. The couplings prevent undesired forward movement of the battens around the sides of the mast with the easing off of the outhaul in an effort to increase camber and thereby prevent the leading ends of the battens from distorting the airfoil shape and disrupting airflow. While those disclosed systems afford certain control of the sail's shape, their capabilities fall far short of maintaining an optimal airfoil shape, particularly at high airspeed and low angles of attack. Additionally, the amount of camber induced by such systems is inextricably tied to the adjustment of the outhaul and downhaul. Furthermore, these prior art devices are unable to provide the capability of independently varying the camber along the sail's span and similarly are incapable of altering the sail's twist. Finally, the prior art devices are unable to prevent decambering of the sail at low angles of attack and high wind velocities.